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template<typename typename >

void

???? = *

//...

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Why do we need decltype?

template<typename typename >

void

auto = *

//...

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Why do we need decltype?

template<typename typename >

void

// Pre-C++11 compiler extension, now obsolete

typedef typeof *

//...

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Why do we need decltype?

template<typename typename >

void

typedef decltype *

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Trailing return type

template<typename typename >

// Does not compile: lhs and rhs are not in scope

decltype(lhs * rhs)

return *

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Trailing return type syntax

template<typename typename >

auto -> decltype(lhs * rhs)

return *

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decltype(expr) : How to resolve the type

(expr == NAME*) ? AS DECLARED :

(expr == lvalue) ? T& :

// rvalue

(expr == xvalue) ? T&& : T //prvalue

* NAME: plain, unparenthesised variable, function - parameter, class member access

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rvalue classification

* xvalue:

- function call where the function’s return value is declared as and rvalue reference

e.g. std:move(x)

- static cast to an rvalue reference

e.g. static_cast<A&&>(a)

- a member access of an xvalue

e.g. static_cast<A&&>(a)).m_x

* prvalue: all other rvalues than above cases

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struct S { S() { m_x = 42; } int m_x; }; int x; const int cx = 42; const int& crx = x; const S* p = new S();

• decltype((x))

• decltype((cx))

• decltype((crx))

• decltype((p->m_x))

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const S foo(); const int& foobar(); std::vector<int> vect = {42, 43};

• decltype(foo())

• decltype(fooobar())

• decltype(vect.begin())

• decltype(vect[0])

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int x = 0; int y = 0; const int cx = 42; const int cy = 43; double d1 = 3.14; double d2 = 2.72;

* decltype(x * y) * decltype(cx * cy) * decltype(d1 > d2 ? d1 : d2) * decltype(x < d2 ? x : d2)

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// C++11, deducing return type is available only for lambdas

template<typename Container, typename Index> auto authAndAccess(Container& c, Index i) -> decltype(c[i]) { authenticateUser(); return c[i]; }

// C++14, deducing is possible for lambdas and all functions

template<typename Container, typename Index> auto authAndAccess(Container& c, Index i) { authenticateUser(); return c[i]; } -> auto makes references in return type ignored

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// C++14

template<typename Container, typename Index>

decltype(auto)

authAndAccess(Container& c, Index i)

{

authenticateUser();

return c[i];

}

-> decltype(auto)follows decltype rule as Scott Meyer explained. Now return value can be produced as a reference

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std::deque<std::string> makeStringDeque();

auto s = authAndAccess(makeStringDeque(), 5);

// C++14

template<typename Container, typename Index>

decltype(auto)

authAndAccess(Container&& c, Index i)

{

authenticateUser();

return std::forward<Container>(c)[i];

}

-> rvalue reference can be dealt with.

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std::deque<std::string> makeStringDeque();

auto s = authAndAccess(makeStringDeque(), 5);

// C++11

template<typename Container, typename Index>

auto

authAndAccess(Container&& c, Index i)

-> decltype(std::forward<Container>(c)[i])

{

authenticateUser();

return std::forward<Container>(c)[i];

}

-> rvalue reference can be dealt with.

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- decltype almost always yields the type of a variable or expression without any modifications.

- For lvalue expressions of type T other than names, decltype always reports a type of T&.

- C++14 supports decltype(auto), which, like auto, deduces a type from its initializer, but it performs the type deduction using the decltype rules.

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• http://thbecker.net/articles/auto_and_decltype/section_05.html

• http://scottmeyers.blogspot.kr/2013/07/ when-decltype-meets-auto.html